1. Field of the Invention
The present invention relates generally to a plasma generating apparatus and more particularly to a plasma generating apparatus wherein a plasma is generated by microwaves and a magnetic field to etch a sample such as a semiconductor wafer or to form a film on the sample.
2. Description of the Related Art
In general, in accordance with the higher integration density of an integrated circuit formed on a semiconductor substrate, etc., it is necessary to reduce the size of circuit elements constituting the integrated circuit and the thickness of wires for connecting these circuit elements, with high reliability.
In an example of the apparatus for manufacturing the integrated circuit, there is employed a plasma generating apparatus wherein a magnetic field is applied in a low-vacuum atmosphere into which a process gas is introduced, and microwaves are applied to the magnetic field to accelerate electrons in a cyclotron, thus generating a plasma. A typical example of this apparatus is an ECR (Electron Cyclotron Resonance) apparatus.
The ECR apparatus is generally divided into a plasma generating chamber and a reaction chamber. A plasma generated in the plasma generating chamber is taken out of the chamber in the form of a beam and is radiated to a sample placed in the reaction chamber directly or via a target. Thus, the sample is etched, or a film is formed on the sample. A coil for generating the magnetic field in the plasma generating chamber is parallel to a major surface of a semiconductor wafer of the sample, and the coil is wound around the plasma generating chamber in a ring-shape.
The plasma generated by the plasma generating apparatus employed in the ECR apparatus has a high density. In addition, since ions travel straight along a magnetic field in a direction in which a magnetic field is attenuated, the plasma has a relatively anisotropic property and a low temperature.
Accordingly, when the ECR apparatus is employed as an etching apparatus, a straight anisotropic beam plasma is radiated onto a major surface of the wafer to carry out fine processing; on the other hand, if the ECR apparatus is employed as a thin-film forming apparatus, burying of contact holes can be effectively performed by virtue of anisotropy in the direction of formation of thin-film.
However, the above ECR apparatus has a two-chamber structure, and the size thereof is large. In order to effectively take out ions from the plasma in the electric field and radiate the ions on the wafer, a high voltage (several hundred voltage) must be applied. In this case, there is problem that the ion energy increases due to the application of high voltage. The consumption of power can be reduced and ion energy is reduced, for example, by placing the sample in the plasma; however, with this structure, the sample absorbs microwaves as heat or high-energy electrons collide with the sample, and therefore the temperature thereof increases. Consequently, the quality and characteristic of the formed film are damaged, and the quality of the film is degraded.
As is well known, when an electric current is supplied to a coil for generating a magnetic field, a magnetic field is formed such that the lines of magnetic force extend from the inside of the coil, run along the outer periphery of the coil, and return to the inside of the coil.
When the sample is situated such that the lines of magnetic force emitted from the inside of the coil are made incident perpendicularly onto the major surface of the wafer, the lines enter the center part of the major surface perpendicularly but do not enter the outer peripheral part of the major surface. Specifically, the lines are inclined from the center of the sample towards the outer periphery thereof.
When the above ECR apparatus is employed for etching, the plasma enters in the direction of the lines of magnetic force, and the lines do not perpendicularly enter the entire surface of the sample. Thus, vertical etching cannot be performed. As a result, fine processing cannot be carried out, owing to side etching effected on the outer peripheral part of the sample and a difference in etching rate (the center part is etched at a higher rate than the outer peripheral part).
In particular, in accordance with the increase in area of the sample, the influence due to the above problems increases.
More specifically, as the area of the semiconductor wafer of the sample increases, the size of the coil increases accordingly and a large electric power for generating a necessary magnetic field is required. For example, when microwaves of 2450 MHz are employed, the magnetic field of 875 gauss is required in the ECR region. When the diameter of the wafer is 10 inches, the size of the magnetic field generating coil increases such that, for example, the inside diameter is 500 mm, the outside diameter is 660 mm and the height is 80 mm. In this case, in order to generate an intense magnetic field of 875 gauss in the entire ECR region, an electric current of 40,000 ampere's turn or more must be supplied to the magnetic field generating. The electric power to be required is 16 KWh or more. In addition, when the diameter of the wafer is 12 inches or more, the size of the apparatus increases and a large power is required.